1. Field of the Invention
The present invention relates to a zoom lens. For example, this is a compact zoom lens which comprises, a long back focus at a wide field angle and an excellent pupil compliance with an illumination system is kept, and the compact zoom lens is preferably used for a high-resolution liquid crystal projector, and a mobile liquid crystal projector in particular.
In addition, the present invention also relates to a zoom lens having high optical performance best suited to a camera (optical apparatus) with a relatively long back focus among a lens shutter camera, video camera, digital camera, etc.
2. Description of the Related Art
Conventionally, there are various proposals on a liquid crystal projector (image projection apparatus) which uses a display element such as a liquid crystal display element to project an image based on the display element onto a screen surface.
Especially, a liquid crystal projector is widely used as an apparatus capable of projecting an image of a personal computer, etc., onto a large screen for meetings and presentations, etc.
A 3-plate type color liquid crystal projection apparatus using three liquid crystal display elements needs to provide a space for arranging elements such as a dichroic prism which combines color light components modulated by a liquid crystal display element and a polarizing plate between the liquid crystal display element and a projection lens, which requires a back focus of a determinate length to be secured with respect to the projection lens.
A projection optical system (projection lens) used for a color liquid crystal projector is required:                to be a so-called telecentric optical system with a pupil on the liquid crystal display element (demagnifying) side arranged at infinity to secure excellent compliance of the pupil with an illumination system so as to minimize an influence of angle dependency of a color combining film provided on the dichroic prism;        when pictures (images) of three color liquid crystal display elements are combined and projected onto a screen, to superimpose pixels of the three colors on one another over the entire screen to prevent characters, etc., on a personal computer from appearing doubled, reducing the level of resolution or deteriorating quality, which requires color shifts (chromatic aberration of magnification) generated by the projection lens to be sufficiently corrected in a wideband of visible light; and        to also correct distortion, preventing the projected image from being distorted on the contours and becoming visually undesirable.        
Furthermore, there is recently not only a demand for a screen with high brightness and an image with high resolution but also a demand for reductions in size and weight of an apparatus with importance attached to maneuverability and portability of a projector provided with a small panel.
As a projection lens for a liquid crystal projector, there is conventionally a proposal on a 6-unit zoom lens which consists of a total of six lens units; first to sixth lens units having negative, positive, positive, negative, positive (or negative) and positive refractive powers in order from the magnifying side (front side) and performs zooming by moving a predetermined lens unit appropriately (e.g., Japanese Patent Application Laid-Open No. 2001-235679 corresponds to U.S. Patent Application Publication No. 2001-0050818).
This 6-unit zoom lens fixes the first and sixth lens units during zooming and moves all the second to fifth lens units in the lens system toward the demagnifying conjugate side (rear side) during zooming from the wide-angle end to the telephoto end, and therefore the overall lens length during zooming is kept constant and is designed to be telecentric with reduced distortion and chromatic aberration during zooming.
As another projection lens for a conventional liquid crystal projector, there is a proposal on a 6-unit zoom lens which consists of a total of six lens units; first to sixth lens units having negative, positive, positive, negative, positive and positive refractive powers in order from the magnifying side (front side) and performs zooming by moving a predetermined lens unit appropriately (e.g., Japanese Patent Application Laid-Open No. 2001-108900). This 6-unit zoom lens fixes the first, fourth and sixth lens units and moves the second, third and fifth lens units in the lens system during variation of magnification from the wide-angle end to the telephoto end, and therefore an overall lens length is kept constant and is designed to be telecentric repressing variations in various types of aberration such as chromatic aberration during variation of magnification.
Together with the demand for further downsizing of a liquid crystal projector, there is currently a strong demand for the ability to realize short-distance projection which constitutes a great merit for a home theater in particular, that is, a liquid crystal projector with a wider field angle.
There is also a demand for a projection lens with a large aperture ratio for the purpose of providing higher brightness for a projected image.
The 6-unit zoom lens disclosed in U.S. Patent Application Publication No. 2001-0050818 arranges a lens unit having a positive refractive power on the most magnifying side (front side), and therefore it is advantageous for correcting distortion. However, since the diameter of a front lens unit, etc., increases and a glass material with a small refractive index is selected for the sixth lens unit added to the most demagnifying side (rear side), there is a tendency that it is difficult to correct aberration of off-axis light beams in particular.